A major source of xylenes is catalytic reformate, which is produced by contacting petroleum naphtha with a hydrogenation/dehydrogenation catalyst on a support. The resulting reformate is a complex mixture of paraffins and C6 to C8 aromatics, in addition to a significant quantity of heavier aromatic hydrocarbons. After removing the light (C5−) paraffinic components, the remainder of reformate is normally separated into C7−, C8, and C9+-containing fractions using a plurality of distillation steps. Benzene can then be recovered from the C7−-containing fraction to leave a toluene-rich fraction which is generally used to produce additional C8 aromatics by toluene disproportionation and/or transalkylation with part of the C9+-aromatics containing fraction. The C8-containing fraction is fed to a xylene production loop where para-xylene is recovered, generally by adsorption or crystallization, and the resultant para-xylene depleted stream is subjected to catalytic conversion to isomerize the xylenes back towards equilibrium distribution. The resultant isomerized xylene stream can then be recycled to the para-xylene recovery unit.
Although benzene and toluene are important aromatic hydrocarbons, the demand for xylenes, particularly para-xylene, outstrips that for benzene and toluene and currently is growing at an annual rate of 5-7%. There is, therefore, a continuing need to develop aromatics production technologies which maximize the production of para-xylene, while minimizing the associated capital and operating costs.